I replaced the fan on my 1000X with a Noctua fan too a few weeks ago, the stock one was incredibly loud and annoying. A lot better now, I'm not sure yet if I should silence it further with one of the included low-noise adapters. It's mostly airflow noise now. The heatsinks on the board are getting quite hot, I might have to make some measurements.
Then I got a second 1000X yesterday. To my surprise, it is way quieter than the first one when I turned it on. No need to replace the fan. Now I get why people are saying the scope is silent enough, but it's odd there is such a difference between them. I don't have anything to measure the noise level, but I'd guess it's half as loud and an order of magnitude less annoying.
Maybe there is a quality problem with the fan? The second scope's serial number is lower than the first one, so I guess it's a bit older.